1. Field of the Invention
This invention relates to an improvement in a control system for an automatic transmission combining a four element torque converter with a planatery gearing arrangement.
2. Description of the Related Art
In a conventional control system for an automatic transmission of a vehicle driven through a fluid-coupling such as a torque converter with a lockup clutch, gear shifts between a plurality of forward and reverse gear ratios are provided selectively changing the engagement state of a plurality of friction elements.
In transmission of the type controlled by electro/hydraulic controls, the sequential engagement and disengagement of the friction elements during a gear shift is controlled by the actuation and deactuation of associated shift control valves. The actuating and deactuation of the shift valves is, in turn, controlled by the energization and de-energization of solenoid valves associated with each of the shift valves.
In transmission control systems of this type, the lock-up clutch is also selectively controlled by the actuation and deactuation of a lockup control valve which, in turn, is controlled by the energization and de-engerization of a lockup solenoid. The actuation and de-actuation of each of the solenoid valves in such a control system is controlled by a central processing unit which receives sensed inputs indicative of vehicle operation such as engine load and vehicle speed.
With an automatic transmission control system of this kind, it is desirable to decrease the weight and space taken up by the transmission control system while improving the efficiency of operation of the transmission.
The present invention seeks to achieve this objective by providing an improved transmission control system and method of operation which uniquely accomplishes the stated goal by using fewer components in the transmission control system. The use of fewer components in the control system of the transmission inherently reduces the weight of the transmission and the space taken up by the transmission control system, thereby increasing its operating efficency while maintaining operational reliability.
It is therefore an object of the present invention to provide an improved electro/hydraulic transmission control system having a reduced number of operative parts, and to provide a unique interrelationship between the existing parts of the transmission control system to produce a system which controls a transmission having a plurality of forward gear ratios and a torque converter lockup or direct drive condition.